The invention relates to the field of color imaging and color separation in an active pixel array.
The use of multiple photodiodes positioned at different depths in a silicon material to operate as color sensors is well known in the art. For example, U.S. Pat. No. 5,965,875 (xe2x80x9cthe ""875 Patentxe2x80x9d) discloses a xe2x80x9ctriple wellxe2x80x9d structure which comprises three photodiodes positioned at different depths in a material. The ""875 Patent is incorporated herein by reference. As shown in the ""875 patent, one photodiode is positioned at a depth of approximately two microns below the surface of the material. At this depth range, the photodiode corresponds to detection of red photons. Another photodiode which corresponds to the detection of green photons is positioned at a depth of approximately 0.6 xcexcm below the surface of the material. Positioned above the green photodiode is a blue photodiode.
In the structure disclosed in the ""875 patent, if one desires to increase the sensitivity of the color detection structure, then the likely approach is to increase the size of the pixel by increasing the doping regions of the diodes. This approach, however, leads to an increase in the capacitance across the junctions in the photodiodes. As the capacitance increases, some of the benefits of increasing the overall area pixel are lost. This is because the amount of charge accumulated in the photodiode is proportional to the capacitance times the voltage. The voltage is equal to the total electrical charge divided by the capacitance. The charge will increase as the area is increased, but at the same time the area of the p/n junction of the diode is increasing. Thus, the capacitance across the p/n junction is also increasing, so the voltage will tend to remain constant even though the overall area of the pixel is being increased. Thus, it can be difficult to increase the sensitivity of the device.
What is needed is photodetector structure which allows for the sensitivity of a photodetector to be increased, by varying the area where the photons are detected without increasing the capacitance in the photodiodes and thereby losing much of the benefit of increasing the detection area.
The present invention is directed to a color-sensing device that provides photocarrier separation and low photodiode capacitance. The method for making the color sensing device starts with forming a silicon structure on a substrate. The silicon structure includes a first region, a second region, and a third region. A barrier region separates the first region from the second region, and another barrier region separates the second region from the third region. A first trench is etched into the structure, such that the bottom of the trench is positioned in the first region, and a second trench is etched into the structure such that the bottom of the second trench is positioned in the second region. In each the three regions a photodiode is formed which detects electrical charges excited by the photons of light with differing wavelengths.
One embodiment of the invention provides a structure which has three regions of the of the same conductivity type. Each of the three regions is separated by a barrier region, which serves to inhibit the flow of electrons between the regions. In one embodiment, the invention includes a silicon structure with a first region in the silicon doped with Pxe2x88x92 dopant, wherein the first region is disposed in a first depth range in the silicon which corresponds to a depth red light penetrates into the silicon. The structure also includes a second region in the silicon doped with Pxe2x88x92 dopant, wherein the second region is disposed in a second depth range in the silicon which corresponds to a depth that green light penetrates into the silicon; and a third region in the silicon doped with Pxe2x88x92 dopant, wherein the third region is disposed in a third depth range in the silicon which corresponds to a depth that blue light penetrates into the silicon. A first trench is formed in the silicon, such that a bottom of the first trench is disposed in the first region. A second trench formed in the silicon, such that a bottom of the second trench is disposed in the second region. A third trench formed in the silicon, such that a bottom of the third trench is disposed in the third region. Adjacent to the bottom of each of the trenches are regions containing N+ dopant which are disposed in the first, second and third regions. The junctions between the silicon regions containing N+ dopant and the Pxe2x88x92 dopant form photodiodes.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and the appended claims.